1. Field of the Invention
In general, the present invention relates to a method and apparatus for detecting multipath images. In particular, when utilizing an active sensor, such as a radar, false "ghost" images, created when the sensor beam reflects from more than one surface before being detected by a receiver, may be mistaken for returns from actual objects. More particularly, the present invention relates to a method and apparatus for distinguishing between false "ghost" images and actual airplanes in an air traffic control environment.
2. Description of the Prior Art
Radar has served as an effective sensor for detecting both cooperative and non-cooperative airborne and surface targets since World War II. As illustrated in FIG. 1, a typical radar system 10 includes a transmitter/receiver 12 which emits RF energy from an antenna 14 that "bounces off" a target A, B and returns to the transmitter/receiver 12. Pulse radar systems, such as Airport Surface Detection Equipment (ASDE) systems determine range R1 to a target A by measuring the time between transmission of a pulse and receipt of the return pulse. The azimuth angle theta is obtained by correlating the return pulse with a position of the antenna 14. The strength of the return is a function of the reflectivity of the target A and numerous other factors such as range, weather, etc.
Accidents involving aircraft and ground vehicles and aircraft ground collisions can occur at busy airports. More than three times as many near-accidents occur on the ground as in the air. In one such accident, an aircraft strayed onto the wrong runway and was struck by a second aircraft, resulting in a loss of life. In another accident, two aircraft collided when one aircraft was cleared to land on an occupied runway.
Medium-range airport surveillance radar, such as the Automated Radar Terminal System (ARTS) is good for detecting and tracking many aircraft within a large volume of airspace. However, such systems do not provide adequate surveillance coverage for ground-resident objects, including aircraft that are in the taxiiing, holding (stopped), takeoff or landing phases of their flight profiles.
ASDE systems can provide high-resolution, short-range, clutter-free, surveillance information on aircraft and ground vehicles, both moving and fixed, located on or near the surface of airport movement and holding areas under all weather and visibility conditions. An ASDE system formats incoming surface detection radar information for a desired coverage area, and presents it to local and ground controllers on high-resolution, bright displays in the airport control tower cab. In addition to ASDE and ARTS systems, other sensor systems such as, for example, secondary surveillance radar (SSR), and global positioning system (GPS) can provide logically disparate parameters in physically disparate locations within the tower cab.
A large, busy airport is an environment having a vast number of possible conflict situations. In such a dynamic environment, the potential for collision between any given aircraft and any one of possible many ground-resident, and nearby airborne, objects may not be recognized until it is too late to avoid the collision.
In particular, the presence of multipath returns are a significant problem at large busy airports. As illustrated in FIG. 2, an ASDE-3 system 20 emits radar radiation. This radar radiation is reflected off an aircraft 22 on the ground and then off stationary ground objects such as a building 24, and projects a false "ghost" image 26 on the runway 28 or a false "ghost" image, which is not on a runway. Conventional ASDE system handle the off-runway situation by always blanking the same area, which means that images which appear in areas other than the runways, are not considered. As a result, in the conventional ASDE system, a multipath ghost image not on a runway, would not be presented to an air traffic controller.
Similarly, the conventional AMASS system generates track data in airways, taxiways, and runways and blanks out everything behind the first track.
What is needed therefore is an apparatus and method that processes all multipath signals and distinguishes between false "ghost" images and actual airplanes in an air traffic control environment.